If where you focus lack sufficient light or contrast, the Speedlite's AF-Assist beam is emitted automatically to help the camera autofocus. The AF-Assist beam is effective from 0.7 to 10 meters (2.3 to 33 feet)
AF-Assist beam activation
Depending on the camera and the active focusing point, either the camera's AF-assist beam or he Speedlite's AF-assist beam is emitted. Speedlite 380EX's AF-assist beam is linkable only to the center focusing point. If the camera has only one focusing point (AF frame) the Speedlite's AF-assist bean is emitted. If the camera has multiple focusing points, the camera's AF-assist beam is emitted instead.
